The hearing aid adjustment to the listening needs of a hearing impaired is traditionally performed in one of the following ways:                a) The signal is split up into a predefined number of frequency bands where each band comprises a frequency sub-range, whereby the attenuation in each frequency sub-range is controlled. This is called the multi-channel approach and n is a fixed number chosen by the manufacturer. The special case when n=1 is called single-channel.        b) The signal is split up in signal analysis path and a signal processing path. Attenuation values are calculated in the analysing path and applied at one single filter in the signal processing path where the input signal gets corrected according to the needs of the user. This is called channelfree processing. The analysis path can be split up in a number of frequency bands but the signal processing path is un-affected by this.        
An example of channelfree processing is disclosed in US patent application publication US 2004/0175011 A1, filed Feb. 24, 2004 incorporated herein as reference.
The effect of using different processing schemes and a different number of channels is the subject of the two below articles:                The preferred Number of Channels (one, two, or four) in NAL-NL 1 Prescribed WDRC Device; Gitte Keidser and Frances Grant; ear & hearing 2001, 22, 516-527.        Benefits of linear amplification and multichannel compression for speech comprehension in backgrounds with spectral and temporal dips. Brian Moore et al. JASA 105 (1) January 1999.        
The shape of the hearing loss and the sound environment may well influence the number of channels chosen as proposed from G. Keidser et al in Ear & Hearing 2001. For example, it is known that for music a one channel processing is superior to a multi-channel approach. References can be found at: Boothroyd, A., Mulheam, B., Gong, J., & Ostroff, J. 1996. Effects of spectral smearing on phoneme and word recognition are discussed in: J. Acoust. Soc. Am, 100, 1807-1818. Here it is shown that using multiple channels results in spectral smearing. Especially for music spectral smearing is a very annoying side effect of signal processing and should be avoided. The same approach applies to speech-understanding but here comfort of venting or noise impact the channel decision.
It can be learned from the above articles that many hearing impaired people prefer the single channel approach, because this approach gives the best listening comfort. The multi-channel approach has however, the benefit that it gives the user a better understanding of speech in noise.
None of these articles propose to change the number of channels dynamically according to the sound environment or the hearing impairment.
The idea of the invention is to provide a hearing aid, which combines the benefits of the various proposed processing schemes. The channelfree implementation actually allows a switching of the number of analysis path channels in dependency of the user or environment demand. Channelfree refers to the audio signal which is only modified in one filter, the signal itself is not sent through multiple filters as in multi-channel approaches nor is it sent through amplification blocks in a number of frequency ranges. The invention also allows switching between Channelfree and multi-channel. This means that the number of channels can be dynamically chosen in the signal path and/or the analysis path.